The invar alloys have been extensively used in all kinds of high-precision mechanical devices, watches, etc. The theoretical background for this temperature independence has been explained only recently.

The long and difficult experiments performed by Guillaume year after year on numerous alloys and above all on nickel steel to determine their expansibility, elasticity, hardness, changeability with age, and stability ultimately led him to the important discovery of the nickel steel alloy known as invar, the temperature coefficient of which is practically zero.